As a device having performance exceeding the limits of that of silicon bipolar transistors, there is a hetero-junction bipolar transistor (hereinafter referred to as "HBT"). The HBT has, as will be clear from FIG. 1 which shows the energy band diagram thereof, a hetero-junction 4 which is formed between an emitter region 2 and a base region 1 to thereby make the energy bandgap of the emitter wider than that of the base. In the figure, 3 denotes a collector region. Attempts have been vigorously made to realize HBTs using compound semiconductors, and it has become possible to realize HBTs as a result, particularly, of the recent progress in epitaxial growth technology or the like.
The feature of HBTs resides in that, since there is substantially no injection of minority carriers from the base into the emitter, it is possible to increase the impurity concentration in the base while maintaining the emitter injection efficiency at a high level. Therefore, it is possible to narrow the base width and lower the internal base resistance. As a result, it is possible to improve the current gain and cut-off frequency of the transistor as compared with the conventional bipolar transistors.
As one technique used to further improve various characteristics of the above-described HBT, for example, a proposition made by Ito et al. in Extended Abstracts (The 46th Autumn Meeting, 1985) 3a-B-9; The Japan Society of Applied Physics may be cited. It is described in this literature that it is possible to reduce the transit time of electrons and increase the current gain by sloping the Al composition in the base layer of an AlGaAs/GaAs HBT. However, in the above-described prior art, the base layer is a heavily-doped p-type layer and therefore it has the problem that the hole mobility is small. Accordingly, lowering in the base resistance is limited, and this leads to restrictions on the high-frequency characteristics.
It is described in Japanese Patent Laid-Open No. 60-10775 that the base resistance is reduced by forming a base region having a superlattice structure and thus shortening the switching time and improving the high-frequency characteristics. This prior art suffers, however, from the problem that the collector injection efficiency is lowered as a result of recombination of carriers inside the base region.